1. Field of the Invention
The present invention relates to a sheet processing apparatus, an image forming system, and a sheet processing method, and more particularly to a sheet processing apparatus that binds sheet-shaped recording media (hereinafter referred to as “sheets” also in the appended claims) such as sheets of paper, sheets of transfer paper and sheets, an image forming system including the sheet processing apparatus and an image forming apparatus such as a copying machine, a printing machine, a facsimile machine, or a digital multifunction peripheral (MFP) having at least two functions of these machines, and a sheet processing method performed by the sheet processing apparatus.
2. Description of the Related Art
There is well known a sheet postprocessing apparatus, a so-called finisher, that temporarily stacks, on a stacking tray, sheets each having an image formed thereon by an image forming apparatus, such as a copier, a printer, or an MFP, and discharged from the image forming apparatus, aligns the sheets, and thereafter binds the sheets using a stapler that uses a metal staple. Such a sheet postprocessing apparatus is in widespread use because it increases convenience and efficiency by automatically binding a large number of copies of sheets each having an image formed thereon.
An example of such a technique is disclosed in Japanese Patent No. 3617936. A sheet processing apparatus according to this technique is configured as follows to avoid decrease in productivity in image formation without increasing complexity, size, and production cost of the apparatus. That is, the apparatus includes a postprocessing tray arranged inside the apparatus, a conveying path to guide a sheet to the postprocessing tray, and a sheet conveying unit to convey a sheet. The apparatus stacks sheets on the postprocessing tray, performs processing such as binding on the sheets, and thereafter discharges the sheets onto an output tray. The apparatus is configured to prevent backward traveling of the sheet at a predetermined position. The apparatus includes a branch path downstream of the predetermined position and a control unit that controls the sheet conveying unit. Under control of the control unit, the sheet conveying unit can move the sheet conveyed past the predetermined position backward and hold the sheet on the branch path. The sheet conveying unit can also convey one or more sheets held on the branch path together with a next conveyed sheet toward the postprocessing tray in a state where the next conveyed sheet is stacked on the one or more sheets.
According to this technique, when performing stapling processing, sheets are temporarily stacked on a stacking tray and aligned. After that, the sheets are stapled and then discharged onto an output tray. However, during a period when the sheets are aligned and stapled, another sheet cannot be stacked on the stacking tray. Accordingly, it has been necessary to temporarily stop output from the image forming apparatus.
In contrast to hand staplers that are generally capable of binding approximately 10 sheets, mainstream models of the sheet postprocessing apparatuses are capable of binding approximately 50 sheets. This number, 50, is the number requested by many users.
However, sheet postprocessing apparatuses (finishers) capable of binding approximately 50 sheets at a maximum are undesirably about the same size as copiers or printers. As a matter of course, such a finisher is not only relatively expensive and requires large space but also consumes much resources and a large amount of energy. In recent years, users are very sensitive to cost, space, and energy and resources consumption, and therefore cost reduction, space saving, and energy and resources saving are required.
Meanwhile, number of sheets which office users bind is small such as approximately five in the majority of cases, and binding as many as approximately 50 sheets is a rare occasion. In spite of that, a user has no other choice but to purchase a sheet postprocessing apparatus capable of binding approximately 50 sheets even when frequency of binding approximately 50 sheets is low, if the user desires to enhance efficiency in sheet processing. In other words, there have been only two choices: purchasing a sheet postprocessing apparatus capable of binding approximately 50 sheets to enhance efficiency, or giving up enhancing efficiency, in which case a user performs manual binding using a hand stapler.
There is a need to provide a sheet processing apparatus capable of low-volume binding while satisfying demands for cost reduction, space saving, resources saving, and energy saving.